This invention relates to a frequency converter and, more particularly, to an improved frequency converter wherein the frequency of an input signal is converted to a predetermined frequency, such as an intermediate frequency (IF), and wherein the possibility of producing a spurious noise or noise due to temperature drift of the circuit components is signficantly minimized.
Frequency converters are well-known to the prior art for use in, for example, amplitude modulation (AM) receivers wherein the information in a received broadcast signal is converted to an IF signal for demodulation. Another use of the frequency converter is, for example, in a video signal recording system wherein chrominance components are converted to a lower frequency in a recording operation and reproduced chrominance components are reconverted to their longitudinal frequency during a playback operation. Various other uses of frequency converters are readily apparent; and for the purpose of the present description, such use will be described in the environment of a radio receiver wherein the received RF signal is converted to an IF signal.
A known type of frequency converter is formed of a local oscillator for generating a local oscillating and a double-balanced type mixer which is supplied with the input signal to be frequency-converted by the local oscillating signal. The local oscillator includes a differential amplifier and the double-balanced type mixer includes a pair of differential amplifiers each of which being connected in cascade with yet another differential amplifier. In use, the high frequency signal which is to be frequency-converted is supplied to the pair of differential amplifiers and the output signal from the local oscillator is supplied to the further differential amplifier. In this manner, as each stage of the further differential amplifier is driven alternately by the local oscillating signal, currents flow through the respective ones of the pair of differential amplifiers so that the signals are multiplied, or mixed, from which the desired IF signal can be derived.
In this type of prior art frequency converter, the output signal supplied by the local oscillator to the doublebalanced type mixer is an unbalanced signal. Unfortunately, this has the result of producing a spurious signal, or noise. Another disadvantage in this type of prior art frequency converter is that the differential amplifiers in the local oscillator and in the double-balanced type mixer, although supplied with DC bias voltages, generally are supplied with such voltages from different bias sources. This requires a relatively complex bias circuit configuration. Also, since separate bias sources are used, the effect of temperature drift on one is not necessarily matched or compensated by the influence of temperature drift of the other. Consequently, noise due to such temperature drive deleteriously affects the frequency-converted output signal.